Regarding buying cheaper batteries opposed to OEM ones, same reason why one would by third party lenses really, they can be cheaper and sometimes even better. As someone already stated, Fuji does not make batteries, they could be rebranded wasabis or any other brand. For some people spending 50 pounds on a battery might not be a big deal, some might have struggled to get the money for the camera in the first place, others just will not pay blindly for what they could get cheaper.

The people that advocate only using OEM batteries go throughout life being ripped off every day. As you said, they can't use 3rd party lenses. They are full of electronics that might damage the camera. Same thing for flashes. Memory cards? Back to the OEM because they electronically interface with the camera and might cause problems.

Can you imagine the rest of their lives. What did that car cost? Better not use oil other than that branded by the auto manufacturer. Need parts? Back to the dealer for OEM parts.

Got a lawn mower, leaf blower, edge trimmer? Only oil and parts branded by the OEM. The list goes on and on.

I suspect almost none of these brand advocates adhere to their own advice. If they do, they make a lot more money than I ever made.